Pin photodetector with mini-mesa contact layer

ABSTRACT

A PIN photodetector includes a first semiconductor contact layer, a semiconductor absorption layer having a larger area than the first semiconductor contact layer, a semiconductor passivation layer positioned between the first semiconductor contact layer and absorption layer, and a second semiconductor contact layer. The semiconductor absorption layer and passivation layers are positioned between the first and second semiconductor contact layers.

RELATED APPLICATION

The present application claims the benefit of U.S. ProvisionalApplication No. 60/467,399, filed May 2, 2003, the entire contents ofwhich are incorporated herein by reference.

BACKGROUND

The present invention generally relates to the field of photodetecton.More specifically, the invention relates to the detection of photonsusing a semiconductor photodetector.

Owing to the known interaction between photons and electrons, advanceshave been made in the field of photodetectors in recent years,particularly in those photodetectors that utilize semiconductormaterials. One type of semiconductor-based photodetector known as an PINphotodetector includes a number of semiconductve materials that servedifferent purposes such as absorption and passivation.

With many types of PIN photodetectors, the sensitivity and reliabilityof photodetectors degrade over time. Further, the photodetectorsexperience general fatigue and wear and tear. It is, therefore,desirable to present a photodetector that maintains high responsivity,high bandwidth, and low dark current over its intended lifetime, as wellas being simple to fabricate.

SUMMARY OF THE INVENTION

The present invention provides a PIN photodetector including a firstsemiconductor contact layer, a semiconductor absorption layer having alarger area than the first semiconductor contact layer, a semiconductorpassivation layer having a larger area than the first semiconductorcontact layer, and positioned between the first semiconductor contactlayer and absorption layer, and a second semiconductor contact layer.The semiconductor absorption layer and passivation layers are positionedbetween the first and second semiconductor contact layers as in FIG. 1.

When the photodetector is in use, the electric field near the center ofthe semiconductor absorption layer is greater than the electric fieldnear the edges of the semiconductor absorption layer as indicated inFIG. 2, and the capacitance of the photodiode is also determined by thearea of the first small semiconductor contact layer. The photodetectormay have a 3 dB bandwidth greater than 20 GHz. in certain embodiments,the photodiode has a dark current behavior that is substantiallyconstant over long time periods (e.g. 20 years) relative to an initialvalue.

Embodiments of the invention may have one or more of the followingadvantages. The configuration has an increased lifetime and improvedtemperature aging characteristics. The first semiconductor contact layerdefines a mini-mesa structure that is advantageous for an enhancedabsorption high performance, high bandwidth PIN. Moreover, thefabrication of the mini-mesa PIN photodetector is simplified since theneed for a p-diffusion step to form a localized p-contact is eliminated.

Other features and advantages will be apparent from the description andfrom the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a PIN structure in accordance with anembodiment of the invention;

FIG. 2 is a graph showing the electric field profile across theabsorption layer of the PIN structure of FIG. 1;

FIG. 3 is a graph showing the dark current behavior for a group ofconventional mesa devices aged at 125° C. with a constant voltage bias;

FIG. 4 s a graph showing the dark current behavior for a group of PINstructures of FIG. 1 aged at 175° C. with a constant voltage bias;

FIG. 5 is a side view of an alternative embodiment of a PIN structure inaccordance with the invention.

DETAILED DESCRIPTION

Referring now to the drawings, a photodetector, in particular, a minimesa PIN photodetector embodying the principles of the present inventionis illustrated therein and designated at 10. As its primary components,the PIN photodetector 10 includes an n+ contact layer 12, a p+ metalcontact layer 14, and a p+ mini mesa 16. An InGaAs absorption layer 22is disposed between the p+ mini mesa 16 and the n+ contact layer 12. Apair of bandgap grading layers 20 bound the InGaAs absorption layer 22.An nid (“not intentionally doped”) passivation layer 18 is also disposedbetween the InGaAs absorption layer 22 and the p+ mini mesa 16. Inparticular embodiments, a passivation layer 24 is disposed on the outersurface of the PIN photodetector 10. The passivation layer 24 may be BCB(benzocyclobutene), silicon dioxide, silicon nitride, or polyimide. An nmetal contact 26 collects electrons and is positioned on the n+contactlayer 12.

Because the mini mesa 16 has a reduced area, the electric fields at theedges of the large n-mesa are substantially reduced, thus reducing thedeleterious effects of any surface states or other surface defects.Furthermore, since the current is also reduced at these surfaces anycharging or interface states at these boundaries is reduced.

FIG. 2 shows schematically the electric field profile across theabsorption layer 22 for a PIN photodetector with a 30 μm mini-mesa 16and a 50 μm outer n-mesa. The field drops to near zero at the edges ofthe outer mesa which shows the passivation characteristics of the PINphotodetector 10.

These effects substantially increase the lifetime and improve the agingcharacteristics of photodetectors, such as APDs and undoped or low dopedPINs, above that of the conventional mesa photodetector devices.

FIGS. 3 and 4 illustrate a comparison between the device agingcharacteristics of a conventional device (FIG. 3) and that of the PINphotodetector 10 (FIG. 4). FIG. 3 shows the dark current behavior for agroup of conventional mesa devices aged at the relatively low agingtemperature of 125° C. with a constant voltage bias. As shown, the darkcurrent increases a factor of 20 times from the initial values in only1500 hours, indicating a rapid degradation of these mesa devices.

FIG. 4, in contrast, shows the dark current behavior for a group ofmini-mesa PIN photodetectors 10 aged at the much higher agingtemperature of 175° C., with a constant voltage bias. As is readilyseen, the dark currents for the PIN photodetectors 10 hold steady totheir initial values with little or no degradation over 5000 hours. Thiscorresponds to a lifetime that is greater than 20 years at normaloperating temperatures, such as, for example, 70° C.

One of the features of the mini mesa PIN photodetector 10 is that thecapacitance of the photodetector is not significantly increased becauseof the larger n-mesa. Consequently, the bandwidth of the PINphotodetector 10 does not differ considerably from the bandwidth of theconvention mesa PINs, as experimentally verified through a series ofdevice measurements using a Lightwave Component Analyzer.

A comparison of the measured electrical bandwidth of the mini mesa PINsand the traditional mesa PINs shows that the 3 dB bandwidth for both a40 micron diameter mini-mesa PIN photodetector 10, and a similar sizedstandard mesa PIN are both about 15 GHz. Therefore, the PINphotodetector 10 has more than adequate bandwidth for OC-192 telecomapplications.

Moreover, the mini mesa PIN photodetector 10 is particularly suitablefor “enhanced” doped PINs, with graded doping concentrations whichgreatly increase the speed and sensitivity of high bandwidth PINs. Insome implementations, the photodetector structure involves a grading ofthe p doping, such that the PIN structure is inverted with the p contacton the top and the n doping is on top, as illustrated as a PINphotodetector 110 in FIG. 5.

The PIN photodetector 110 includes a p+ contact 112, such as InAlAs, ann+ metal contact 114, and an n+ mini mesa 116. In certain embodiments,the n+ mini mesa 116 is InAlAs. An absorption layer 122 which may be lowdoped or nid InGaAs, is disposed between the n+mini mesa 116 and the p+contact 112. A pair of bandgap graded layers 120, is disposed above andbeneath the absorption layer 122. The graded p+ layer 124 is disposedbetween the absorption layer 122 and the p+ contact 112 such that thedoping concentration of the graded p+ layer 124 increases with proximityto the p+ contacts 112. An nid passivation layer 118, preferably InAlAs,is disposed between the n+mini mesa 116 and the upper bandgap gradedlayer 120. A passivation 126 is disposed on the surface of the enhancedPIN 110. The passivation layer 126 may be, for example, BCB(benzocyclobutene), silicon dioxide, silicon nitride, or polymide. The pmetal contact 128 is positioned on the p+ contact layer 112. Thisstructure permits the graded p absorption layer to be as wide as thelarge outer contact mesa, and still have a small mini-mesa n contact toreduce capacitance and increase the bandwidth.

A simple etching process with a stop etch layer can be used to fabricatethe aforementioned PIN photodetectors 10 or 110. These simple etchedmini mesa structures can be reproducibly grown and fabricated, and arehighly uniform over the entire wafer. The full structure is growninitially and then it is etched down to define a small localized minimesa contact region which controls the relevant capacitance area, thusresulting in a low capacitance, high speed PIN. Thus, this design doesnot require a diffusion step to define the small top contact, and istherefore simpler and produces photodetectors which are more highlyuniform over the entire wafer.

Note that in the PIN structures 10, 110, the high surface field near thetop of the structure is very well controlled by the high bandgappassivation layers 18 and 118. As mentioned previously, these structuresare high speed since the low capacitance is determined by the area ofthe small mini mesa diameter and not the large noncritical isolationmesa.

The above and other implementations of the principles of the inventionare within the scope of the following claims.

1. A PIN photodetector comprising: a first semiconductor contact layer;a semiconductor absorption layer, the first semiconductor contact layerhaving a smaller area than the semiconductor absorption layer; asemiconductor passivation layer positioned between the firstsemiconductor contact layer and the semiconductor absorption layer; asecond semiconductor contact layer, the semiconductor absorption layerand passivation layer being positioned between the first and secondsemiconductor contact layers; a first bandgap grading layer positionedbetween the semiconductor passivation layer and the semiconductorabsorption layer and a second bandgap grading layer positioned betweenthe semiconductor absorption layer and the second semiconductor contactlayer; and wherein the second bandgap grading layer is directly adjacentto the second semiconductor contact layer.
 2. The photodetector of claim1 wherein the semiconductor absorption layer is InGaAs.
 3. Thephotodetector of claim 1 wherein the passivation layer is InAlAs.
 4. Thephotodetector of claim 1 wherein the wherein the first semiconductorcontact layer is a p-type and the second semiconductor contact layer isan n-type.
 5. The photodetector of claim 1 wherein the wherein the firstsemiconductor contact layer is an n-type and the second semiconductorcontact layer is a p-type.
 6. The photodetector of claim 5 wherein thefirst and second semiconductor contact layers are InAlAs.
 7. Thephotodetector of claim 1 further comprising a second semiconductorpassivation layer positioned about the first semiconductor passivationlayer and the semiconductor absorption layer.
 8. The photodetector ofclaim 1 further comprising a first metal contact positioned adjacent tothe first semiconductor contact layer and at least one second metalcontact positioned adjacent to the second semiconductor contact layer.9. The photodetector of claim 8 wherein the first metal contact is ap-type and the second metal contact is an n-type.
 10. The photodetectorof claim 8 wherein the first metal contact is an n-type and the secondmetal contact is a p-type.
 11. The photodetector of claim 1 wherein theelectric field near the center of the semiconductor absorption layer isgreater than the electric field near the edges of the semiconductorabsorption layer.
 12. The photodetector of claim 1 wherein thecapacitance of the photodiode is determined by the area of the firstsemiconductor contact layer.
 13. The photodetector of claim 1 whereinthe photodiode has a dark current behavior that is substantiallyconstant relative to an initial value.
 14. The photodetector of claim 13wherein the photodiode has a dark current behavior that is substantiallyconstant relative to an initial value over a time period greater than2000 hours.
 15. The photodetector of claim 1 wherein the photodiode hasa lifetime that exceeds twenty years.
 16. The photodetector of claim 1,where other semiconductors such as InP or other binary or tertiary III-Vsemiconductors are used.
 17. The photodetector of claim 1 wherein thesecond bandgap grading layer further comprises a graded p+ layer.
 18. Amethod of fabricating a PIN photodetector comprising: providing a lowersemiconductor contact layer; depositing a semiconductor absorptionlayer; depositing a semiconductor passivation layer; depositing orfabricating an upper semiconductor contact layer having a smaller areathan the semiconductor absorption layer; depositing a first bandgapgrading layer between the lower semiconductor contact layer and thesemiconductor absorption layer and depositing a second bandgap gradinglayer between the semiconductor absorption layer and the semiconductorpassivation layer; and wherein the first bandgap grading layer isdirectly adjacent to the lower semiconductor contact layer.
 19. Themethod of claim 18 wherein the semiconductor absorption layer is InGaAs.20. The method of claim 18 wherein the passivation layer is InAlAs. 21.The method of claim 18 wherein the wherein the lower semiconductorcontact layer is an n-type and the upper semiconductor contact layer isa p-type.
 22. The method of claim 18 wherein the wherein the lowersemiconductor contact layer is a p-type and the upper semiconductorcontact layer is an n-type.
 23. The method of claim 22 wherein bothsemiconductor contact layers are InAlAs.
 24. The method of claim 18further comprising depositing a second semiconductor passivation layerabout the first semiconductor passivation layer and the semiconductorabsorption layer.
 25. The method of claim 18 using other semiconductorssuch as InP or other binary or tertiary III-V semiconductors.
 26. Themethod of claim 18 wherein the first bandgap grading layer furthercomprises a graded p+ layer.